Electrochemical gas sensors typically contain two or three electrodes—a sensing and combined counter/reference for two electrode sensors, or a sensing, counter and reference for three electrode sensors. The principles of such sensors-have been described in ‘Liquid Electrolyte Fuel Cells’, B S Hobbs, A D S Tantram and R Chan-Henry, Ch 6 in ‘Techniques and Mechanisms In Gas Sensing’, Eds P T Moseley, J O W Norris and D E Williams, pub Adam Hilger 1991.
Certain changes in the characteristics of any of the electrodes can result in degradation of sensor performance. For example, contamination, poisoning or changes in oxidation state of the working electrode can change its activity relative to target gases. Similar effects on the counter electrode can result in it needing to be polarized to a greater extent to maintain the required bias voltage between the sensing and reference electrodes. This in turn can be problematic as the control electronics may not be able to provide sufficient voltage, or the counter electrode may be driven to a potential where undesirable electrochemical effects occur, such as evolution of gases, or production of species which may diffuse to the reference or working electrodes.
This latter effect is particularly problematic due to the typically close proximity of the three electrodes in practical commercial sensors. The reference electrode used in commercial electrochemical gas sensors is typically a ‘pseudo reference’ and as such its reference potential can be affected by factors such as poisoning, or varying concentrations of species in the surrounding electrolyte. For example, changes in dissolved oxygen concentration around a platinum pseudo reference electrode can change the reference potential. Changes in the reference potential result in an equal change in the working electrode potential since the latter is controlled potentiostatically relative to the former. As a result, changes in the reference potential can change the activity of the working electrode as well as its surface state and long term stability resulting in changes in cross sensitivity, or in extreme cases unwanted evolution of gases such as hydrogen or oxygen.
The evolution of bubbles of gas on any of the three electrodes can also result in reduction of contact area with the electrolyte. On the sensing electrode the reduction in active area can result in reduction in gas sensitivity. On the counter electrode the reduction in contact area can result in the need for higher polarization voltage. On the reference electrode this effect might not be expected to have any affect as no significant current is drawn. However it is known that in practice changes in the surface area of reference electrodes can have effects on sensor performance.
A specific example of the problem is exemplified with a non-consumable electrochemical oxygen ‘pump’ sensor. On this sensor oxygen is consumed on the sensing electrode and evolved on the counter electrode. The reference electrode needs to be in electrochemical contact with the electrolyte and the other electrodes. As a result the oxygen concentration in the vicinity of the reference electrode may vary over time. It is desirable to control the oxygen concentration in the vicinity of the reference electrode or to maintain it below a certain limit. Too high a concentration results in an anodic shift in the reference potential, which causes a corresponding anodic shift in the working electrode potential. This gives rise to an immediate reduction in the activity of the working electrode, and may also result in longer term drift in the working electrode performance.
It is known from the prior art that an electrode can be ‘cleaned up’ electrochemically by biasing it/passing current through it. It is also known that metal electrodes (for example, platinum) can be ‘cleaned’ by cycling their potential in acid to oxidize then strip surface oxides.